A construction machine, such as a hydraulic excavator, is typically equipped with an engine, which serves as a power source, and an operator's cab, which is provided with operation switches for performing various works, including starting or stopping the engine. Unlike automobiles, construction machines are normally designed to be operated with a common engine key so that an operator can operate a plurality of construction machines with a single type of key. This, however, makes construction machines vulnerable to theft or tampering. In order to solve this problem, a construction machine is usually equipped with an anti-theft system.
FIG. 11 shows a block diagram of an example of conventional anti-theft systems. FIG. 12 is a control block diagram of an anti-theft unit of said anti-theft system. FIG. 13 is a system block diagram of a hydraulic excavator, which is employed herein as an example of construction machines.
As shown in FIG. 11, a starter switch 10 is provided. The starter switch 10 serves to start or stop the engine by switching the electric power, which is supplied from a battery 9 as a result of operation of the engine key 8. A main relay 11, which is adapted to be energized by the starter switch 10, is connected to an engine start-up relay 12 and a solenoid valve relay 13. The main relay 11 is adapted to be turned on or off by operating the starter switch 10 so as to supply electric power to various components of the main body of the construction machine, which are not shown in the drawings.
Connected to the solenoid valve relay 13 is a hydraulic locking switch 14 to be operated by an operator when the machine is being driven. The hydraulic locking switch 14 is for ensuring safety by being kept in the off position when the machine is not being operated.
A starter motor 15 for starting up the engine is connected to the engine start-up relay 12. A solenoid-operated directional control valve 16 for controlling the direction of pressurized pilot oil is connected to the solenoid valve relay 13. Numeral 17 denotes a ground wire.
The anti-theft system mainly comprises an anti-theft unit 1, an antenna 6, and an identification tag 7 (the term ‘identification’ will hereinafter be abbreviated as ‘ID’). The antenna 6 is incorporated in the starter switch 10. The ID tag 7 is for identifying an individual and attached to an engine key 8.
The anti-theft unit 1 includes a control unit 2, a power supply unit 3, and relays 4,5, which are adapted to be controlled by output from the control unit 2.
FIG. 12 is a control block diagram of the control unit 2. The control unit 2 is provided with an ID code determining unit 20 and an ID code storage unit 21 connected to the ID code determining unit 20. The ID code storage unit 21 is for identifying an individual machine.
By means of radio communication between the antenna 6 and the ID tag 7, the ID code of the ID tag 7 is read into the control unit 2. The control unit 2 is adapted to receive ACC contact signals from the starter switch 10; when the ACC contact of the starter switch 10 is on, the ID code determining unit 20 functions to collate the ID code of the ID tag 7 with the ID code stored in the ID code storage unit 21, and, according to the result of collation, the relays 4,5 are activated.
Referring to FIG. 13, pressurized oil fed from a main pump 23, which is adapted to be driven by the engine 22, is controlled and distributed to hydraulic cylinders 25 by control valves 24. The circuit pressure in a pump discharge circuit is limited by a relief valve 26.
A pilot pump 27 driven by the engine 22 discharges pressurized pilot oil, which is fed to the aforementioned solenoid-operated directional control valve 16. The solenoid-operated directional control valve 16 determines whether the pilot oil should be fed to solenoid-operated directional control valves 28a, 28b, 28c or discharged into a tank. The solenoid-operated directional control valves 28a, 28b, 28c are remote control valves for controlling the control valves 24. The pilot pressure is held at a certain constant level by a pilot pressure relief valve 29.
Next, the function of the example of conventional anti-theft systems described above is explained hereunder.
Referring to FIG. 11, when the engine key 8, to which an ID tag 7 is attached, is inserted into the starter switch 10 and turned from the OFF position to the ACC position, radio communication is activated between the antenna 6 and the ID tag 7 so that the ID code associated with the ID tag 7 is read into the anti-theft unit 1.
The ID code determining unit 20 of the control unit 2 shown in FIG. 12 collates the ID code of the ID tag 7 with the ID code stored in the ID code storage unit 21. If the two ID codes match, an ON signal is output from the ID code determining unit 20 so that the engine start-up relay 12 and the solenoid valve relay 13 are energized via the relays 4,5.
When the starter switch 10 is turned to the start-up position ST in the state described above, an electric current is supplied to the starter motor 15 via the main relay 11 and the engine start-up relay 12 so that the engine 22 starts up.
Should the operator turn on the hydraulic locking switch 14, the solenoid-operated directional control valve 16 switches over from chamber n to chamber a so as to feed the pressurized oil from the pilot pump 27 to the remote control valves 28. By operating the remote control valves 28, the directional control valves 24 can be changed over so that the pressurized oil from the main pump 23 is fed to the appropriate hydraulic cylinders 25.
Because of the function described above, an operator can drive the construction machine by using an engine key 8 to which a registered ID tag 7 is attached.
Should an attempt be made to start the engine with an engine key 8 having an unregistered ID tag 7 or having no ID tag 7 at all, neither the engine start-up relay 12 nor the solenoid valve relay 13 will be energized, because the ID codes do not match. As a result, it becomes impossible to start the engine or operate the construction machine.
When the anti-theft unit 1 of a conventional anti-theft system is installed at a place that allows easy installation, it is easy to disable the anti-theft unit 1 by removing the unit or cutting a cable.
Another problem of the conventional system described above lies in that the system permits start-up of the engine by merely checking ID codes; it is vulnerable to theft, because the engine 22 can be started by connecting the plus line of the battery 9 directly to the starter motor 15.
In order to solve the above problems, an object of the invention is to provide a construction machine which is less vulnerable to theft compared with conventional construction machines.